1. Field of the Invention
Embodiments of the present invention generally relate to the treatment of toxic gases and, more particularly, to systems and methods for treating landfill gas using landfill leachate.
2. Background of the Invention
Landfills are a source of toxic gases known as landfill gases. The major components of landfill gas are methane (CH4), carbon dioxide (CO2) and nitrogen (N2). There are also minor amounts of oxygen (O2), other organic compounds, halides, and hydrogen sulfide (H2S). At present, landfill gas can, for example, be vented to the atmosphere, burned in boilers, flared, and/or used as a natural gas substitute after the CO2 is removed from landfill gas. In the United States and most of the developed world, environmental regulations require landfills to recover landfill gas in order to minimize landfill gas emissions.
Landfills are characteristically odorous facilities, as the incoming waste is odorous. Placing the incoming waste in the landfill and covering it with soil typically mitigates odor, but does not eliminate it. A substantial contributor to odor is water, which facilitates aerobic and/or anaerobic decomposition of landfill trash, thereby promulgating and exacerbating odor problems. During anaerobic decomposition, sulfate salts such as gypsum from discarded wallboard in the landfill waste, can produce hydrogen sulfide, a particularly odorous material. The more water present, the more odorous substances are generally produced. Therefore, landfill operating procedures typically encourage minimizing water contact with landfill waste in order to mitigate odor problems at the landfill. For decades, landfills have been operated as dry as possible, even though the incoming trash may, for example, contain approximately 25 weight percent water.
When a landfill cell is completed (i.e., filled), the contained waste is a loaf-like mass, typically wrapped in a plastic barrier and entombed in several feet of soil. The base of the loaf-like mass typically includes a leachate collection system used to collect liquid draining from the landfill contents, while the outer surface prevents entry of moisture from the environment. One exemplary leachate collection system is described in U.S. Publication No. 20060222464, published Oct. 5, 2006, entitled Aerobic and Anaerobic Waste Management Systems and Methods for Landfills, which is incorporated herein by reference. Despite these measures, the encapsulated waste is still wet, perhaps 15 weight percent water on average.
Once the cell closure is completed, the internal chemistry starts to operate, producing landfill gas and leading eventually to methane production. Initially, oxygen in the system is consumed via oxidation of the trash, thereby producing carbon dioxide and water. The gas that is produced is evacuated by the gas collection system and the liquid water is evacuated via the leachate collection system. When all the oxygen has been consumed, the internal chemistry becomes anaerobic, thereby producing a chemically reduced gas instead of a chemically oxidized gas.
Anaerobic decomposition of landfill waste allows the sulfate salts to be reduced, producing small amounts of hydrogen sulfide which, in turn, may cause several problems, such as odor problems, corrosion of gas recovery hardware, increasing SOx emissions from flaring or other combustion processes, and possible adverse health consequences for workers. The odor threshold for hydrogen sulfide is extremely low (0.05 to 0.1 ppmv), and levels of hydrogen sulfide above 10 ppmv are considered toxic. Moreover, levels of hydrogen sulfide above 1000 ppmv (0.1 V %) in a breathing zone can rapidly lead to unconsciousness and death. Thus, worker health and safety issues may require special attention at sites with high hydrogen sulfide.
Hydrogen sulfide removal from gas and liquid streams is a developed technology, generally involving metal ion catalysis. In general, these hydrogen sulfide removal processes are designed to remove the hydrogen sulfide gas contained in a process stream. For example, a gas stream exiting from a landfill can be delivered to a gas treatment plant for purification. These process schemes can remove hydrogen sulfide in gas streams, thereby reducing or eliminating corrosion problems and/or combustion exhaust gas emission problems.
Exemplary conventional hydrogen sulfide treatment systems used to mitigate hydrogen sulfide in landfill gas are SulfaTreat® (SulfaTreat*, Chesterfield, Mont.) and Mini-Cat™ (Lo-Cat®, Merichem Company, Houston, Tex.). In general, these systems use fixed-bed or batch type granular hydrogen sulfide reactants contained in pressurized vessels. Hydrogen sulfide is removed from a gas stream via a chemical reaction with reactants in the vessel. However, all these techniques tend to be costly. Further, these techniques create another problem relating to the disposal of spent material.
We have determined that a need exists for a cost-effective process to reduce toxic and offensive gases, such as hydrogen sulfide, that are generated by and from landfills.